The invention relates generally to a non-invasive pulse oximetry system and specifically to an improved method and apparatus for determination of blood constituents using a heart rate signal from a remote monitor.
Pulse oximeters typically measure and display various blood characteristics including but not limited to blood oxygen saturation of hemoglobin in arterial blood, volume of individual blood pulsation supplying the flesh, and the rate of blood pulsations corresponding to the heartbeat of the patient. This is typically done by detecting the amount of red and infrared signals absorbed by oxygen in the blood. A problem with non-invasive pulse oximeters is that the pulse rate which appears in the oximeter signal may be subject to irregular variants that interfere with the detection of blood flow characteristics including but not limited to motion artifact. Motion artifact can be caused by the patient's muscle movement proximate to the oximeter sensor and/or movement of the sensor. In fetuses this can include labor, maternal breathing, and maternal motion. In adults, movement of the patient's finger, ear or other body part to which an oximeter sensor is attached, may cause spurious pulses similar to pulses caused by arterial blood flow. These spurious pulses, in turn, may cause the oximeter to process the artifact waveform and provide erroneous data. This problem is particularly significant with infants, fetuses, or patients that do not remain still during monitoring.
A second problem exists in circumstances where the patient is in poor condition and the pulse strength is very weak. In addition to poor patient condition, low pulse strength may also be due to the choice of sensor site. In continuously processing the optical signal data, it can be difficult to separate the true pulsatile component from artifact pulses and noise because of a low signal to noise ratio. Inability to reliably detect the pulsatile component in the optical signal may result in a lack of the information needed to accurately calculate blood constituents.
U.S. Pat. No. 4,911,167 to Corenman et al. and U.S. Pat. No. 4,928,692 to Goodman et al. both describe a method and apparatus for increasing the accuracy of the pulse oximetry measurements using an electrocardiogram ("ECG") waveform. The ECG waveform 100 shown in FIG. 1 comprises a complex waveform having several components that correspond to electrical heart activity. The R-wave portion 102 of the waveform is typically the steepest wave, having the largest amplitude and slope, and may be used for indicating the timing of cardiovascular activity. The improved method described in U.S. Pat. Nos. 4,911,167 and 4,928,692, hereinafter referred to as the C-Lock process, refers to the use of the ECG signal to 1) gate data entries corresponding to a heart beat into a buffer, and 2) to trigger the search for the maximum and minimum values of the optical signals. The C-Lock process improves oximeter accuracy by using the ECG R-wave as a trigger to overlay optical pulses to cancel noise.
By synchronizing the optical signal to the occurrence of successive R-waves, it becomes possible to add the corresponding successive portions of a detected optical signal together so that the periodic information (optical pulses) corresponding to the arterial pulse in each portion of the detected optical signal add in phase. The optical information from a number of periods is added together, with the beginning of each period being determined by the detection of the R-wave. In this way, the maximum and minimum of the optical signal should be lined up with each other in each period and added together to give a cumulative maximum and minimum. This enables precise identification of the maximum and minimum of the signal, and thus allows calculation of the oxygen saturation at that point. Because aperiodic signals have different impulse shapes, duration, height, and relative time of occurrence within each portion, and are not synchronized with heart activity, they do not add in phase. Rather they add in a canceling manner whereby the weighted sum is spread across the relative time frame of the composite portion. Cancellation of the aperiodic signals cancels signal noise resulting in a cleaner optical waveform. This allows the oximeter to track the pulses more reliably and compute a more accurate blood oxygen saturation value with fewer dropouts.
Although the C-Lock process improves oximeter accuracy, an ECG signal is not always available. For example, to monitor fetal ECG heart activity, a scalp electrode is typically attached to the fetus. However, a large portion of deliveries in the United States and virtually all deliveries in Japan do not have access to fetal scalp electrode equipment. Instead, the fetal heart rate is monitored by external non-invasive means such as doppler ultrasound. Although the doppler ultrasound equipment provides heart rate information, it does not output an ECG waveform. Because no ECG waveform is available, the C-Lock process has no periodic R-wave trigger to synchronize optical pulses. Thus, the C-Lock process cannot be used to improve the accuracy of the blood oxygen saturation calculation where an ECG waveform (a fetal scalp electrode) is not available.
A method and apparatus for improving the accuracy of the blood oxygen saturation calculation in an oximetry system where a direct contact ECG waveform signal is not available and which decreases susceptibility to motion artifact is needed.